Cardiac arrhythmia, such as atrial fibrillation, occurs when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm. Important sources of undesired signals are located in various tissue regions in or near the heart, for example, the atria and/or and adjacent structures such as areas of the pulmonary veins, and left and right atrial appendages. Regardless of the sources, unwanted signals are conducted abnormally through heart tissue where they can initiate and/or maintain arrhythmia.
Procedures for treating arrhythmia include surgically disrupting the origin of the signals causing the arrhythmia, as well as disrupting the conducting pathways for such signals. More recently, it has been found that by mapping the electrical properties of the heart muscle in conjunction with the heart anatomy, and selectively ablating cardiac tissue by application of energy, it is possible to cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions.
In a two-step procedure—mapping followed by ablation—electrical activity at points in the heart is typically sensed and measured by advancing a catheter containing one or more electrical sensors into the heart, and acquiring data at a multiplicity of points. These data are then utilized to select the target areas at which ablation is to be performed.
A typical ablation procedure involves the insertion of a catheter having a tip electrode at its distal end into a heart chamber. A reference electrode is provided, generally taped to the patient's skin. Radio frequency (RF) current is applied to the tip electrode, and flows through the surrounding media, i.e., blood and tissue, toward the reference electrode. The distribution of current depends on the amount of electrode surface in contact with the tissue, as compared to blood which has a higher conductivity than the tissue. Heating of the tissue occurs due to its electrical resistivity. If the tissue is heated sufficiently, cellular and other protein destruction ensues; this in turn forms a lesion within the heart muscle which is electrically non-conductive.
A generally-straight catheter works well, for example, when ablating a line of block in the atria. However, for tubular regions in or around the heart, this type of catheter is cumbersome, skill dependent, and time consuming. For example, when the line of block is to be made about a circumference of the tubular region, it is difficult to manipulate and control the distal end of a straight catheter so that it effectively ablates about the circumference. In current practice a line of block is accomplished by maneuvering the catheter from point to point and is highly dependent on the skill of the operator and can suffer from incomplete isolation of target areas such as the pulmonary vein ostia. However, done well, it can be very effective.
Catheters with circular ablation assemblies (or “lasso-type” catheters) are known. This type of catheter comprises a catheter body having at its distal end an ablation assembly with a preformed generally circular curve with an outer surface and being generally transverse to the axis of the catheter body. In this arrangement, the catheter has at least a portion of the outer circumference of the generally circular curve in contact with the inner circumference or ostium of a tubular region in or near the patient's heart, e.g., a pulmonary vein. However, one drawback with catheters of this type may be the relatively fixed size or circumference of the circular ablation assembly, which may not match the circumference of the tubular region undergoing treatment.
Further, the variance in anatomy observed between subjects makes it difficult for a “one size fits all” approach.
Ablation catheters with expandable assemblies are also known. Such catheters have a circumferential ablation element includes an expandable member with a working length that is adjustable from a radially collapsed position to a radially expanded position. This catheter employs an equatorial band that circumscribes the outer surface of the working length and is adapted to ablate tissue adjacent thereto when actuated by an ablation actuator. However, like most catheters with expandable members, the expandable member is a balloon structure that is inflated with a pressurized fluid source. Inflation of the balloon undesirably restricts blood flow. Added complications may also arise when a balloon is forced to seat in the ostium near the treatment region, such as a pulmonary vein.
Also known is a basket catheter having a basket-shaped electrode array with a mechanism for expanding and retracting the electrode array. The basket assembly has a plurality of spines connected at their proximal and distal ends to an expander that is movable longitudinally to expand and contract the basket-shaped electrode. While this assembly can accomplish circumferential ablation, it may be better suited for mapping and other diagnostic procedures in the chamber areas of the heart. Furthermore, wire spines of basket assemblies can in certain circumstances move or shift relative to each other, rendering the structure of the basket assemblies less stable than desirable.
Accordingly, a need exists for an improved catheter that is particularly useful for circumferential ablation in or near the ostium of tubular regions of the heart. It is desirable that the ablation assembly has a sufficiently stable framework yet be sufficiently pliable and flexible to enable optimal circumferential contact of tissue surrounding an ostium with minimal disturbance or obstruction to blood flow in the region.